An original, parsimonious model of the human circadian system based on two interacting oscillators, reproduces the sleep-wake (SW) and core temperature (Tc) patterns of free-running subjects (subjects isolated from time cures, selecting their own SW cycle), including both synchrony and desynchrony. Correspondences with data suggest that all model parameters except one can be considered invariant for a subject over the course of even a long experiment. The only labile parameter is the intrinsic period of the rhythm generator governing SW. By fitting the model to data, the resulting parameter set becomes a quantitative descriptor of the subject. A population of normal subjects can be defined in terms of the range and expected values of these descriptors. Abnormal subjects can be defined in terms of deviations from the morms. Ordinarily environmental zeitgebers (z) appear to act directly only on the SW oscillator, not on the stable, "deep" oscillator which dominates Tc. The deep oscillator is linked to the environment through coupling to SW and consequently is slow in adjusting to imposed timing shifts as in transmeridional flight (i.e., jet lag). The strength of z can be estimated experimentally by altering its period and finding the range of entrainment for SW or Tc. The modeling will be integrated with ongoing human experiments (Psychoneuroendocrine Rhythms: Aging and Sleep Disorders, currently funded by NIMH and NTA Dr. Elliot D. Weitzman, Montefiore Hospital, Principal Investigator). It will be used to design protocol for experiments involving zeitgebers (their strength, period, phase shifts and duration) and interpret the results as generated. The experiments will be tests of hypotheses which are modeled in advance. (Examples: the tie course of internal phase adjustments; the implications of circadian abnormality in manic-depression; the implication of rhythm splitting in short-activity-rhythm desynchrony.) The model will also be used to test the relevance of ongoing primate experiments (squirrel monkey) to the human circadian system.